Just recently updated to macOS Tahoe 26.4 Beta (25e5207k). After update and reboot, errors popped up "macOS can’t repair the disk “Pegasus32”.
Both my systems can only be read and cannot write, repair or anything at this point.
Might be isolated, but no issues on other systems not on this beta.
This sounds like a new variant of a problem that has occurred many times recently. Your Pegasus32 uses disks formatted HFS+, right? Apple's extensive and ongoing security through the Secure Enclave tests file systems before allowing them access to macOS. Apple's focus seems to be on APFS. Unfortunately it seems Apple often is negligent in testing how this testing works with HFS+-formatted disks. In the past what has been found to work is to turn off security for the macOS system, then use Disk Warrior to rebuild and replace the HFS+ directory. After doing this, the disk and file system should mount and function proprerly.
Of course, "proprerly" is a subjective judgement. Running your system with Apple's extensive security turned off many might not consider proper. When reported to Apple, Apple replied that this is not considered a security issue.
This was the status last summer of issues which seem to be as you describe. Apple's Secure Enclave seems to be undergoing constant development, often blocking HFS+ file systems, then being modified to permit them.
