External drives · Choosing a drive
Can I use a second partition or volume on my Mac's internal drive?
Short answer: No. Volumes on your internal drive share the same storage, so moving files between them frees nothing. Storage Studio marks internal volumes not suitable — the fast drive must be a separate, external drive.
No. Any volume on your Mac's internal drive is marked not suitable, because moving files from one part of the same disk to another doesn't free any space — APFS volumes on one disk share the same pool of storage. The whole point of the fast drive is to put data on a different physical drive.
The drive list in Choose Drive shows your built-in disk with an internal icon so you can see it, but its verdict will always be not suitable, and Transfer Drive and Backup Drive never list it at all.