One of my backup disks has problems. I keep this disk (BU2) at another location and do a backup when I'm there. The last backup was from before the upgrade to El Capitan. At home I have another backup disk (BU1), which took a very long time to complete after this backup. BU2 had to backup 130GB. After several hours only 30GB was done. It could have been only 30MB, I'm not sure anymore. I tried a second time, but no change.
I took the disk home and erased it, made a new partition, did a check. This all seemed fine. Then I linked the disk to Time Machine, checking the encryption option. I enter the password, and after clicking OK I get this message:
There was an error preparing the Time Machine backup disk BU2. Some information was unavailable during an internal lookup.
I tried this several times. Is this a problem with Time Machine, or is this disk dying? How can I do a thorough check? Disk Utitily is not what it was anymore. What is the best way to check it?