First, check if the file system of your external HDD is supported by macOS Monterey. If macOS isn't compatible with this file system, the disk you inserted is not readable by this Mac. To use an external drive on macOS Monterey, you'd better format your external drive into APFS file system.
However, formating will erase all data on your drive. Thus, you need to back up all files on it first. Since the disk you insert is not readable on your MBP, you need to find another way to open and access files on this drive.
iBoysoft Data Recovery for Mac is a secure and well-known data recovery tool capable of recovering data from unreadable or inaccessible external hard drives, SD cards, USB flash drives, etc. This utility is compatible with macOS 13/12/11/10.15/10.14/10.13/10.12 and OS X 10.11. Now, do the following to recover data from your unreadable external drive and fix the disk unredable error on Mac by reformatting:
Step 1: Free download and install iBoysoft Data Recovery for Mac.
Step 2: Open iBoysoft Data Recovery for Mac and insert the external HDD into your Mac.
Step 3: Select the unreadable HDD and click "Search for Lost Data".
Step 4: Preview the found files for free.
Step 5: Check the boxes before the files you want to recover, then click the Recover button and save them to your Mac or a different storage device.
After your data is recovered, you can format the external drive on Mac to APFS to see if the unredable issue has been resolved.
