I am also having the same problem. if no link provided for item files, import works well though I have tried only for two items.
When adding links to the files, files are not added at all and next items are skipped.
The error log files ay as below:
2015-06-29T08:27:48+02:00 ERR (3): exception 'Omeka_Storage_Exception' with message 'Unable to store file.' in C:\xampp
\htdocs\omeka-2.3\application\libraries\Omeka\Storage\Adapter\Filesystem.php:119
Stack trace:
#0 [internal function]:
Omeka_Storage_Adapter_Filesystem->store('C:\\Users\\IIM\\Ap...', 'original/8a7b21...')
#1 C:\xampp\htdocs\omeka-
2.3\application\libraries\Omeka\Storage.php(67): call_user_func_array(Array, Array)
#2 C:\xampp\htdocs\omeka-
2.3\application\models\File.php(424): Omeka_Storage->__call('store', Array)
#3 C:\xampp\htdocs\omeka-2.3\application\models
\File.php(424): Omeka_Storage->store('C:\\Users\\IIM\\Ap...', 'original/8a7b21...')
#4 C:\xampp\htdocs\omeka-
2.3\application\models\Job\FileProcessUpload.php(23): File->storeFiles()
#5 C:\xampp\htdocs\omeka-2.3\application\libraries
\Omeka\Job\Dispatcher\Adapter\Synchronous.php(25): Job_FileProcessUpload->perform()
#6 C:\xampp\htdocs\omeka-
2.3\application\libraries\Omeka\Job\Dispatcher\Default.php(136): Omeka_Job_Dispatcher_Adapter_Synchronous->send
('{"className":"J...', Array)
#7 C:\xampp\htdocs\omeka-2.3\application\models\File.php(203): Omeka_Job_Dispatcher_Default-
>send('Job_FileProcess...', Array)
#8 [internal function]: File->afterSave(Array)
#9 C:\xampp\htdocs\omeka-2.3\application
\libraries\Omeka\Record\AbstractRecord.php(277): call_user_func(Array, Array)
#10 C:\xampp\htdocs\omeka-2.3\application
\libraries\Omeka\Record\AbstractRecord.php(550): Omeka_Record_AbstractRecord->runCallbacks('afterSave', Array)
#11 C:\xampp
\htdocs\omeka-2.3\application\models\Item.php(335): Omeka_Record_AbstractRecord->save()
#12 C:\xampp\htdocs\omeka-
2.3\application\models\Builder\Item.php(204): Item->saveFiles()
#13 C:\xampp\htdocs\omeka-2.3\application\libraries
\globals.php(567): Builder_Item->addFiles('Url', 'http://localhos...', Array)
#14 C:\xampp\htdocs\omeka-2.3\plugins
\CsvImport\models\CsvImport\Import.php(720): insert_files_for_item(Object(Item), 'Url', 'http://localhos...', Array)
#15 C:
\xampp\htdocs\omeka-2.3\plugins\CsvImport\models\CsvImport\Import.php(588): CsvImport_Import->_addItemFromRow(Array)
#16 C:
\xampp\htdocs\omeka-2.3\plugins\CsvImport\models\CsvImport\Import.php(336): CsvImport_Import->_importLoop(0)
#17 [internal
function]: CsvImport_Import->start()
#18 C:\xampp\htdocs\omeka-2.3\plugins\CsvImport\models\CsvImport\ImportTask.php(39):
call_user_func(Array)
#19 C:\xampp\htdocs\omeka-2.3\application\libraries\Omeka\Job\Dispatcher\Adapter\Synchronous.php(25):
CsvImport_ImportTask->perform()
#20 C:\xampp\htdocs\omeka-2.3\application\libraries\Omeka\Job\Dispatcher\Default.php(150):
Omeka_Job_Dispatcher_Adapter_Synchronous->send('{"className":"C...', Array)
#21 C:\xampp\htdocs\omeka-2.3\plugins\CsvImport
\controllers\IndexController.php(393): Omeka_Job_Dispatcher_Default->sendLongRunning('CsvImport_Impor...', Array)
#22 C:
\xampp\htdocs\omeka-2.3\plugins\CsvImport\controllers\IndexController.php(135): CsvImport_IndexController-
>_dispatchImportTask(Object(CsvImport_Import), 'start')
#23 C:\xampp\htdocs\omeka-2.3\application\libraries\Zend\Controller
\Action.php(516): CsvImport_IndexController->mapColumnsAction()
#24 C:\xampp\htdocs\omeka-2.3\application\libraries\Zend
\Controller\Dispatcher\Standard.php(308): Zend_Controller_Action->dispatch('mapColumnsActio...')
#25 C:\xampp\htdocs\omeka-
2.3\application\libraries\Zend\Controller\Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object
(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#26 C:\xampp\htdocs\omeka-2.3\application\libraries
\Zend\Application\Bootstrap\Bootstrap.php(105): Zend_Controller_Front->dispatch()
#27 C:\xampp\htdocs\omeka-2.3\application
\libraries\Zend\Application.php(382): Zend_Application_Bootstrap_Bootstrap->run()
#28 C:\xampp\htdocs\omeka-2.3\application
\libraries\Omeka\Application.php(79): Zend_Application->run()
#29 C:\xampp\htdocs\omeka-2.3\admin\index.php(28):
Omeka_Application->run()
#30 {main}