Actually, it's not recommended to handle those files by hand. Per
crontab man page:
Each user can have their own crontab, and though
these are files in
/var/spool/cron/crontabs, they are not
intended to be edited directly.
/var/spool are considered temporary/working, that's why they probably get deleted during an upgrade, though a closer look at the
cron package's upgrade scripts may shed some light on this.
Anyway, it's always a good practice to back up your cron entries or keep them in a file in your home directory.
I assume you're using
crontab -e to create crontab files on the fly. If so, you can get a "copy" of your crontab file by doing
crontab -l. Pipe that to a file to get a "backup":
crontab -l > my-crontab
Then you can edit that my-crontab file to add or modify entries, and then "install" it by giving it to crontab:
This does the same syntax checking as