Is using mdadm's RAID-1 of 2 partitions (one on laptop's internal HDD, one on external HDD) a good idea.

I want the system to work as RAID-1 if both drives are present, work as regular volume (degradad RAID-1) if external HDD is unplugged and quickly resync when I plug external HDD again.

Questions:

1. Is it a good idea?
2. Will write-intent bitmap be enough for this task or I need something else?
3. Should I consider doing it at filesystem level (3b. if yes, how?).

Basic requirements are:

1. Quick resync when I re-add the external drive (provided I hasn't changed that partition).
2. More or less consistent data on the removed drive if I remove it not during write/resync operation.

If I remove the drive during resync I expect the data to be somewhat inconsistent, but expect quick resync completion when I re-add it again. E.g. I want the the remaining drive to track what is changed (there can be a lot of changes) and that sync back only those parts that need it.