Extended sectors. Storage arrays format their drives with 520 or 528 bytes per sector so that a checksum or T10 Protection Information travels with every block. A SAS host adaptor reports the block size faithfully and the operating system declines to use it: Windows shows 0 GB and offers to initialise, Linux shows no usable capacity, a PERC reports an incompatible sector size. The forum's fix, sg_format to 512 bytes, works by writing every sector, and the drive is then readable and empty. The bench images at 520 or 528 and strips the extra bytes from the image.
4Kn. A 4Kn drive has 4,096-byte sectors and no 512-byte emulation. Older controllers, older BIOSes and some NAS units cannot address it and report the wrong capacity or none. The model string says which format the drive is: HGST and WD strings ending AL4 or N6 rather than AL5 or E6, Seagate 4Kn suffixes, a 4Kn note on the label. It needs a host that speaks 4Kn, and the bench's imagers do.
Locked SED. A self-encrypting drive whose controller was replaced is detected, sometimes with its capacity, sometimes as locked, and returns nothing readable until the controller's key is presented. The label says SED, TCG, FIPS or ISE; the PERC says Secured or Locked. It needs the key. Failed SSD controller. An SSD whose firmware has halted may still be detected and report 0 GB with a medium-format-corrupted sense code; Dell's LT-series did exactly that at 40,000 hours. It is assessed at controller level, model permitting, and the free look says what is possible.