A drive appearing in File Explorer proves that Windows can use its presented volume. It does not prove that the underlying hardware is eligible for a Storage Spaces pool. Treat these as separate observations before changing a working disk.

Start with an inventory of the proposed drives, enclosures and connection paths. Record what is already stored on each drive and where its independent backup is kept. This investigation does not require creating a pool or accepting a format prompt.

Inspect the physical presentation

Microsoft says Storage Spaces needs at least two additional drives beyond the Windows system drive. USB can be supported, but an enclosure may report its disk as removable, hide several physical disks behind a RAID interface, or present an unsuitable shared identity through a hub.

Inspect the disk itself in Device Manager or Disk Management, rather than only its drive-letter volume. Record the identity and whether Windows reports it as Fixed or Removable where that information is exposed. Compare the number of physical disks you expected with those Windows can actually distinguish.

If the enclosure hides those details, use its exact model documentation or ask its manufacturer how it presents the disks to Windows Storage Spaces. An enclosure advertising several bays is not sufficient evidence that every bay is independently available to the operating system.

Microsoft explains that reinstalling Windows or changing drivers and power settings cannot remove an eligibility limitation created by the hardware's presentation. Avoid repeating software repairs when that is the documented cause.

Choose the next step without risking the existing data

Separate the findings into usable volumes, identifiable physical disks and pool-eligible devices. If only the last category is missing, report that narrow result rather than describing the drive as failed.

Do not switch an enclosure's RAID mode experimentally while it holds the only copy of files. Establish its data-preservation requirements before any hardware-mode change. Likewise, verify an exact replacement enclosure's supported behaviour before purchasing it.

Keep the existing storage arrangement until compatibility, backup and the eventual pool-creation plan are understood. A clear eligibility finding can prevent unnecessary erasure while still identifying which part of the planned setup needs changing.

Sources: Microsoft Storage Spaces eligibility.