"Invalid sibling link" on external HD

The only tool that will fix that error is Disk Warrior but DW is not compatible with Catalina. It should work with any earlier version of macOS. The alternative is to reformat the disk. The former will cost $120; the latter is free but destructive to your files.

BD,

I have come across that error many times, and it isn't one you chalk up to bad RAM. Bad software, maybe. Can also be caused by the occasional glitch or doing an abnormal shutdown while the machine is writing to the disk. At one time DW had a pretty good discussion about the error. I don't know if they still do. Nevertheless, it is a directory-related problem. It happens less often these days since macOS now takes a great effort to write files to the disk, contiguously. Hence, fewer sibling links keeping track of where the next or last block was written. In my experiences with the error only DW could fix the directory. Disk Utility had no idea what an invalid sibling link was. I also think such an error is rarer on SSDs than it was on slower HDDs from the past.

Just my 2 cents worth.

Yes, the drive can continue to be used but not until the problem is fixed by DW or by reformatting the drive. The error is not an indication of a failing or dead drive.

As BDaqua has noted the old Verify was a completely useless operation. If a drive was giving you serious problems but had not died, then you wanted to reformat it, not verify it. Verify did not mark bad sectors. Disk Utility did not actually ever do that because it could not perform a true low-level format as was done in a factory. It used to be possible back when Macs used SCSI drives but disappeared when Apple switched to IDE drives.

Old-time SCSI drive formatters could do high and low level formatting, therefore, it could check for, mark, repair bad physical sectors on a drive. With IDE drives only the factory can do a low-level format or you would spend money for special low-level formatting software. Users could only do high-level formatting on IDE drives. That's what we do using Disk Utility. The only kind of errors a user can fix on a drive would be soft errors. If an IDE drive develops true hard errors, then hope it occurs while the drive is still under warranty. Drive makers usually provide shorter warranties in order to sell at a lower price point. But you may be able to buy the exact same drive with a 5-year warranty if you are willing to pay a premium for the drive. Even 3-year warranties are getting a little harder to find. If a new drive only comes with a 90-day or one-year warranty, then you would be wise not to buy it. Always remember that the reliability of the drive is typically limited by its warranty. Research that has been conducted by Google and others has shown that the typical server drives used last no more than 3 years on average. Home use is not as heavy, so the same drive will last longer. I have several older HDDs I bought ten years ago that are still going strong. But I think that is not a typical experience. Plan on replacing a HDD every five years or so.

Hi... verify wouldn't tell you anyway, you'd need an integrity tester, bit mostly that error is software or flaky RAM, not HDD/SSD.

For cloud backup might try DropBox or pCloud.