I recently put together a new whitebox and everything's running fine. However, one of the disks where ESXi itself is installed might be going bad (it's recycled from another system). Is there a fsck equivalent within the virtually non-existent Service Console to do this?
No, there is not - the filesystems on the disk are FAT-based file systems, for which there is no fsck. Also, much of the data is read from the .tgz files found in the bootbank partitions, and is then loaded into memory and used as a ramdisk.
The point is more that there's no need for one. As I stated, ESXi uses the FAT filesystem for storing the bootbank files and the files for Tools, the Client install, etc. The .tgz files then get unpacked into a ramdisk at boot time. Since FAT doesn't have a fsck associated with it - just scandisk from the old DOS days - and since the ramdisk filesystem is created at each boot, there's little need for fsck, anyway. Furthermore, VMFS3 has its own internal checking routines that don't require fsck.
Scandisk? That's a bit new fangled and modern for FAT, CHKDSK was the king of DOS disk scanners.
What if there is some data corruption on the FAT that causes the images to unpack badly into the Ramdisk? There's always the need for a filesystem checker imo, even if it's only needed in the most unlikely of circumstances.
Sorry, getting my DOS/Windows utils mixed up :-).
If the FAT gets corrupted, ESXi tries to boot from the alternate boot bank. If that fails, it warns you of this and you'll need to use the Install media to do a Repair on the installation.