Why does my U.2 drive not show up in a U.3 bay?

1. 1.List every PCI device with the kernel driver bound to it. This tells you whether the bay is on a direct NVMe path or cabled to a Broadcom tri-mode HBA or RAID card.

   lspci -k -nn

   ReadingDrives on a direct path bind to the nvme driver. If the bays are cabled to a Broadcom tri-mode card, the drives sit behind that card's driver and do not appear as NVMe controllers here; go to the tri-mode path below.
2. 2.On a direct path, check whether the drive linked on PCIe at all.

   lspci -nn | grep -i 'Non-Volatile memory controller'

   ReadingEach directly attached NVMe drive appears as a class [0108] "Non-Volatile memory controller" line. A drive missing here never linked on PCIe: suspect the bay wiring, the cable or the slot mode, not the filesystem.
3. 3.List the NVMe devices the kernel NVMe driver registered.

   nvme list

   ReadingEmpty with drives inserted means either no PCIe link on a direct path, or the drives are behind a tri-mode controller, whose firmware translates SCSI to NVMe so its drives are reached through the controller's driver and tools. Do not conclude the drive is dead from this alone.
4. 4.Check whether the NVMe driver or the Broadcom HBA or RAID driver probed anything at boot or when the drive was inserted.

   dmesg | grep -i -E 'nvme|mpt3sas|megaraid'

   ReadingNo message at all on insertion is consistent with no link. We hold no source that maps specific message text to a U.2 or U.3 mismatch, so do not read meaning into a particular line without the kernel and driver version in hand.
5. 5.Rule out a cause that is not a U.2 or U.3 mismatch at all. If your board has combo (MCIO) ports, check their mode in BIOS: one reader's U.3 drives were missing from nvme list on a Gigabyte MZ73-LM0 because its MCIO ports defaulted to SATA; setting them to x4x4 in BIOS fixed it, with the same cable and drives that worked on another board (thread t5).

1. 1.Ask the controller which drives it sees. This is the authoritative list for anything behind a Broadcom tri-mode card.

   storcli /c0/eall/sall show

   ReadingA drive absent here while physically inserted points at the slot, the cable or the drive type (for example a U.2 drive in a tri-mode-only slot, or a cable that is not a U.2 enabler cable) rather than the OS.
2. 2.Check what the OS is presented through the SCSI layer.

   lsscsi -t -s

   ReadingLists SCSI devices with transport and size, and NVMe devices from lsscsi 0.30 on (add -N to hide NVMe). Use it to see whether a drive behind the tri-mode controller is presented at all.
3. 3.Check the cable part number against the controller's cable guide. Broadcom's 9400 series reaches NVMe through specific "U.2 Enabler" cables (05-50061-00 to 05-50065-00). For the 9500 series Broadcom lists separate direct-attach cables: 05-60005-00 "x8 8654 to 2xU.2 Direct" and 05-60006-00 "x8 8654 to 8xU.3 Direct". A generic SAS cable, or a generic SFF-8643-to-SFF-8639 cable, is not one of them; a reader on a tri-mode 9400-16i saw no drive on any port with such a cable (thread t2).
4. 4.On a Supermicro U.2 backplane with a Supermicro tri-mode card, check the card's cable mix and the backplane configuration. Supermicro: "Each tri-mode controller card must operate in a single NVMe or SAS/SATA mode." Every cable on one card must be NVMe or every cable SAS/SATA, the type cannot be switched without a power cycle, and NVMe use requires the backplane's I2C slave address 0x66h and Gen 2 CPLD register definitions.
5. 5.Find out from the backplane documentation how this slot is wired: a separate U.2 (Quad PCIe) x4 path, or only the shared tri-mode link. The connector alone does not tell you. SFF-TA-1005 Appendix B.1 shows a backplane giving a slot a separate Quad PCIe x4 path alongside SAS/SATA, an ordinary U.2 slot layout, and on the Lenovo ThinkSystem SR630 V3 each backplane slot has its own U.2 x4 or U.3 x1 setting. The spec's example in Table 6-15: a UBM backplane with a U.3 drive connector advertises SFF-TA-1001 PCIe, SAS/SATA and empty as its supported types, not the Quad PCIe (U.2) type.
   ReadingIf the slot has only the shared tri-mode path, a U.2 drive will not link there (SFF-TA-1001 Table 1-1, "No link"). Move the U.2 drive to a slot with a U.2 x4 path, or use a U.3 drive in this one.

1. 1.On the ThinkSystem SR630 V3 each backplane slot has a U.2 x4 or U.3 x1 mode, and Lenovo states "By default, the backplane setting is U.2 x4 mode." U.3 drives on a tri-mode controller are not detected until the slot is set to U.3 x1. On other models, look for the equivalent per-slot setting in that model's own documentation; the SR630 V3 entry is the one we have read. The change takes effect only after a DC power cycle, which powers the whole server off: shut workloads down cleanly and plan the downtime before you apply it.

   Lenovo XCC Web GUI: Storage > Detail > Backplane icon > select slots > Apply, then DC power cycle

   ReadingU.3 drives on a tri-mode controller need U.3 x1, where each NVMe drive runs on a PCIe x1 link to the controller. Lenovo's entry covers U.3 drives in tri-mode only; the default mode is named U.2 x4.
2. 2.If the controller is a ThinkSystem RAID 940 adapter, check which mode it is cabled for. Lenovo: "NVMe U.2 drives are not supported with Tri-Mode." The 940-16i's separate NVMe x4 configuration supports only U.2 drives, and "a x4 connection requires different cabling than a Tri-Mode connection", so moving between the two is a cabling change, not only a setting.

1. 1.List the backplanes iDRAC knows, then the physical disks it sees. Use the first output to identify which backplane the server actually has; add --refkey <backplane FQDD> to the second to list one backplane's disks.

   racadm storage get enclosures -o ; racadm storage get pdisks -o

   ReadingA drive missing from pdisks while seated, or flagged in the iDRAC log with an invalid or unsupported drive type, is consistent with the bay not being wired for that drive type; it can also mean a failed or unseated drive.
2. 2.Read any "invalid drive type" message as a slot-type problem, with care. A UBM backplane reports which drive types each slot supports and which type is installed, and the drive's PRSNT#, IfDet# and IfDet2# pins distinguish a U.2 drive from a U.3 drive, so a controller or BMC can flag a present drive as an unsupported type instead of showing an empty bay. A Dell Community thread (t4) shows iDRAC reporting "Invalid drive type detected in bay 1, slot 1" for a SAS drive in a PCIe SSD backplane bay. We found no Dell document that ties that message to a U.2 drive in a U.3 bay.

1. 1.Know what should happen. SFF-TA-1001 requires a U.3 drive to work in a U.2 slot. A U.3 slot grounds pin S15 (HPT0); a U.2 slot leaves it unconnected. The U.3 drive reads that pin at power-on and defaults to U.2 lane assignments when it is not grounded. The PCIe lanes sit on different pins in the two layouts.
2. 2.If a U.3 drive is not seen through an adapter or cable that works with U.2 drives, suspect the adapter or cable grounding S15: the drive then switches to its U.3 lane layout and may not link on a path wired for U.2. Try the drive in the bay directly or through a different adapter or cable. In the TrueNAS thread (t1) this was a reply's hypothesis, not a confirmed fix.
3. 3.Check the server vendor's support list as well as the spec. Lenovo lists U.3 drives as not supported in the RAID 940-16i's NVMe x4 (U.2) configuration, although SFF-TA-1001 requires U.3 drives to work in U.2 slots. The two statements disagree and we do not resolve it: "not supported" is a qualification statement, not a measured failure, and server support depends on the vendor's own qualification.

SFF-TA-1001 Rev 1.1, section 1: "This specification does mandate that SFF-TA-1001 devices are compatible with both Quad PCIe slots and SFF-TA-1001 slots." and "This specification does not mandate nor imply any SFF-TA-1001 slot compatibility with Quad PCIe devices." Quad PCIe is the U.2 layout. Its Table 1-1 gives "No link" for a Quad PCIe (U.2) drive at x1, x2 and x4 in every SFF-TA-1001 slot width, and notes that the no-link case can be detected out of band through the IFDET2 signal.

The spec requires it, and Lenovo (for the Kioxia CM6-R) and KIOXIA (for its CD6 and CM6) describe their U.3 drives as backward compatible with U.2. Lenovo's RAID 940 guide, however, lists U.3 drives as not supported in the 940-16i's NVMe x4 (U.2) configuration. Those sources disagree; we print both. A server vendor's support list reflects its own qualification, so check it for your platform.

It depends on the controller and backplane vendor. Broadcom says its 9500 adapters are compatible with existing U.2 backplanes and lists a separate U.2 direct-attach cable; Supermicro supports its tri-mode cards on its U.2 backplanes with an all-NVMe or all-SAS/SATA rule per card and a backplane configuration requirement; Lenovo's RAID 940 does not support U.2 drives in tri-mode.

Quarch, which makes SFF-8639 test equipment, describes building one for test use and states that U.2 drives do not work in U.3 because the data lane pin-out does not match. We have not found a vendor document for a production adapter of this kind.