No. 1 -- wget ftp://mediapc/core.db.tar.gz

# wget ftp://mediapc/core.db.tar.gz
--2010-06-18 09:46:24--  ftp://mediapc/core.db.tar.gz
           => `core.db.tar.gz'
Resolving mediapc... 10.0.0.2
Connecting to mediapc|10.0.0.2|:21... connected.
Logging in as anonymous ... Logged in!
==> SYST ... done.    ==> PWD ... done.
==> TYPE I ... done.  ==> CWD not needed.
==> SIZE core.db.tar.gz ... done.
==> PASV ... done.    ==> RETR core.db.tar.gz ... 
No such file `core.db.tar.gz'.

No. 2 -- Comment out all repos besides [core], enable your local server and just one other mirror, and then save the output of pacman -Sy --debug.

Core is already up to date, so I've repeated this using extra too, both with and without my local server.

* Core with local and internet servers:

# pacman -Sy --debug
debug: config: attempting to read file /etc/pacman.conf
debug: config: new section 'options'
debug: config: HoldPkg: pacman
debug: config: HoldPkg: glibc
debug: config: SyncFirst: pacman
debug: config: new section 'core'
debug: setlibpaths() called
debug: option 'cachedir' = /var/cache/pacman/pkg/
debug: registering sync database 'core'
debug: config: including /etc/pacman.d/mirrorlist
debug: config: attempting to read file /etc/pacman.d/mirrorlist
debug: adding new server URL to database 'core': ftp://mediapc
debug: adding new server URL to database 'core': ftp://mirror.internode.on.net/pub/archlinux/core/os/i686
debug: config: finished parsing /etc/pacman.d/mirrorlist
debug: config: finished parsing /etc/pacman.conf
debug: registering local database
:: Synchronizing package databases...
debug: using 'core.db.tar.gz' for download progress
debug: HTTP_PROXY: (null)
debug: http_proxy: (null)
debug: FTP_PROXY:  (null)
debug: ftp_proxy:  (null)
debug: mtimes are identical, skipping core.db.tar.gz
 core is up to date
debug: unregistering database 'local'
debug: unregistering database 'core'


* Extra with local and internet servers:

# pacman -Sy --debug
debug: config: attempting to read file /etc/pacman.conf
debug: config: new section 'options'
debug: config: HoldPkg: pacman
debug: config: HoldPkg: glibc
debug: config: SyncFirst: pacman
debug: config: new section 'extra'
debug: setlibpaths() called
debug: option 'cachedir' = /var/cache/pacman/pkg/
debug: registering sync database 'extra'
debug: config: including /etc/pacman.d/mirrorlist
debug: config: attempting to read file /etc/pacman.d/mirrorlist
debug: adding new server URL to database 'extra': ftp://mediapc
debug: adding new server URL to database 'extra': ftp://mirror.internode.on.net/pub/archlinux/extra/os/i686
debug: config: finished parsing /etc/pacman.d/mirrorlist
debug: config: finished parsing /etc/pacman.conf
debug: registering local database
:: Synchronizing package databases...
debug: using 'extra.db.tar.gz' for download progress
debug: HTTP_PROXY: (null)
debug: http_proxy: (null)
debug: FTP_PROXY:  (null)
debug: ftp_proxy:  (null)
debug: mtimes are identical, skipping extra.db.tar.gz
 extra is up to date
debug: unregistering database 'local'
debug: unregistering database 'extra'


* Extra with only internet server:

# pacman -Sy --debug
debug: config: attempting to read file /etc/pacman.conf
debug: config: new section 'options'
debug: config: HoldPkg: pacman
debug: config: HoldPkg: glibc
debug: config: SyncFirst: pacman
debug: config: new section 'extra'
debug: setlibpaths() called
debug: option 'cachedir' = /var/cache/pacman/pkg/
debug: registering sync database 'extra'
debug: config: including /etc/pacman.d/mirrorlist
debug: config: attempting to read file /etc/pacman.d/mirrorlist
debug: adding new server URL to database 'extra': ftp://mirror.internode.on.net/pub/archlinux/extra/os/i686
debug: config: finished parsing /etc/pacman.d/mirrorlist
debug: config: finished parsing /etc/pacman.conf
debug: registering local database
:: Synchronizing package databases...
debug: using 'extra.db.tar.gz' for download progress
debug: HTTP_PROXY: (null)
debug: http_proxy: (null)
debug: FTP_PROXY:  (null)
debug: ftp_proxy:  (null)
debug: connected to mirror.internode.on.net successfully
downloading extra.db.tar.gz...
debug: database dir '/var/lib/pacman/sync/extra/' does not exist, creating it
debug: sync: new mtime for extra: 1276786645
debug: unregistering database 'local'
debug: unregistering database 'extra'