Disc space usage

One of our build servers recently ran out of disc space while trying to
copy an entire database. This led me to investigate the database cluster,
which is stored on a RAID array with a total size of 1TB. Running a query
to list all databases and their sizes did not add up to the amount of
space being used by Postgres, so I had a look at the pgsql/base directory.
It appears that there are a few large directories that do not correspond
to any database. I wonder if these have been left behind accidentally by
Postgres.

Here are the database directories:

Size (kB) Directory Database

32 pgsql_tmp
4352 11510 template0
4368 1 template1
4464 11511 postgres
5368 30103627 xav-userprofile-test
6096 8088167 outerjoins-userprofile-12.0-copy
8676 30103406 xav-test
10052 31313164 common-tgt-items-kmr-modmine
19956 1108178 modmine-3-preview-18-feb-2008
89452 14578911 common-tgt-items-kmr
118940 9952565 production-xav-13
201192 1257481 common-tgt-items-gtocmine-rns
296552 7040137 common-tgt-items-flyminebuild
1557160 9843085
1699624 18456655 common-src-items-flyminebuild
3376096 278561
3995276 9064702 production-unimine-pride-beta5
8528136 1257482 gtocmine-rns
40815456 29233051
42278196 27473906
47112412 28110832
47913532 32728815 production-flyminebuild:ensembl-anopheles
60519524 32841289 production-flyminebuild:go
67626328 27377902
69513844 32856736 production-flyminebuild:flybase-dmel-gene-fasta
74289908 32938724 production-flyminebuild:pubmed-gene
75786720 32941684 production-flyminebuild:biogrid
77361800 32944072 production-flyminebuild:update-publications
80160256 32947141 production-flyminebuild:create-references
81333908 32574190 flybasemine-production
86356140 12110825
87544200 33049747 production-flyminebuild

So on this server, the wasted space takes up 276GB, which is not
acceptable. I believe that if we re-initialise the cluster and re-create
the databases, these directories would disappear. Taking a look at the
directory 12110825, all the files inside were last accessed several
months ago. So, I have a few questions:

1. Is this space used for anything, or is it just abandoned? Is this a
bug?
2. How do I reclaim this wasted space in a safe manner?
3. How do I prevent this happening again?

Matthew

--
An ant doesn't have a lot of processing power available to it. I'm not trying
to be speciesist - I wouldn't want to detract you from such a wonderful
creature, but, well, there isn't a lot there, is there?
-- Computer Science Lecturer