Chicken-egg: uncorrectable checksum error prevents RAID1 rebalancing

[Tom Hunt <tomdicksonhunt@gmail.com>]

I've been running for a week or two using a single-drive 6TB btrfs
volume. For some of this time, the machine running had bad memory,
which led to various checksum errors. For most of these, I just
deleted the relevant file and reacquired it (the errors fortuitously
never occurring in files which were not easily replaceable). However,
there currently remains a single error which does not appear to be in
any file:

# btrfs scrub status /
scrub status for 85f5b744-f68c-4194-aa90-d6fe238115a3
      scrub started at Fri Jan 22 09:49:02 2016 and finished after 11:55:08
      total bytes scrubbed: 4.27TiB with 1 errors
      error details: csum=1
      corrected errors: 0, uncorrectable errors: 1, unverified errors: 0

# dmesg
(...)
[52841.310422] BTRFS warning (device dm-0): csum failed ino 515 off 15118336 csum 2629660496 expected csum 54021641
[52841.335656] BTRFS warning (device dm-0): csum failed ino 515 off 15118336 csum 2629660496 expected csum 54021641
[95071.256448] BTRFS: bdev /dev/mapper/rootvol_1 errs: wr 0, rd 0, flush 0, corrupt 11, gen 0
[95071.256532] BTRFS: unable to fixup (regular) error at logical 4450167468032 on dev /dev/mapper/rootvol_1

I've searched for ino 515, and the file there does not have any
apparent error (can read the whole thing without problem; deleting and
recreating it does not make the error go away). The error is, of
course, uncorrectable, because it's a single-drive volume. However,
having put in a second drive, the balance filter to convert to raid1
fails because of the I/O error.

How do I deal with this?

-- 
Tom Hunt